Diet and health: I doubt whether there is any field in
science where there is so much opportunity and incentive for medical
charlatans to forge for themselves a lucrative career. Why? I can
identify three key reasons. First, we all eat, so there’s a
potentially huge market. Second, it’s fairly obvious that what we
eat, and how much of it, has a bearing on our general well-being.
Third, scientists don’t know all that much about the relationship
between diet and health – certainly, far less than you’d expect
them to considering the importance of this subject.

Let’s begin by exploring this third theme. Medical
ignorance is an open door to bogus health ‘experts’, who operate
outside the rules of the scientific and medical establishment,
but tend to like to subserve some of the establishment respectability
in peddling their advice or wares. Some of these charlatans even go to
the extent of calling themselves Dr, when their ‘qualification’ is
a PhD obtained by correspondence course from a non-accredited
university – this is only a very small step up from buying a
doctorate off the internet without any study at all.

Now I’m not claiming that science has a monopoly on
all ‘truth’ or even all useful medical information. Science can
and does get things wrong; there exist good doctors and bad ones. But
the scientific system has at its core a solid way of verifying the
accuracy of information, by experimental test and then peer review of
all published work. In medicine it’s important to know whether
treatments work, and the only way this can be done is by verification
by trial and/or experiment. We all have a tendency to fool ourselves;
the scientific method saves us from this.

Many ‘experts’ on diet and health operate outside
the scientific system, yet they make claims about their therapies or
advice that are couched in scientific terms. These often sound
plausible enough to non-experts, but very few of these claims are
backed up by proper evidence and a lot of them are essentially
untestable. I’m not suggesting that these diet experts are all
corrupt and evil – they likely, in many cases, genuinely believe
what they are saying, and want to help people. Yet what they do is
extremely unethical and undermines evidence-based medicine.

Does this all sound a bit rant-like? Let me explain
what I mean by this. Essentially, they are duping people, whether they
recognize this or not, because they are using science talk to
establish credibility, masquerading as part of the medical
establishment, while not paying the price of the entry ticket –
experimental evidence subjected to peer review. By spouting untested
claims and unverifiable theories, they are muddying the waters for
genuine medics and confusing consumers.

Why is it we know so little about the relationship
between diet and health? It’s largely because it’s so hard to
investigate properly. The consequences—for good or ill—of diet
typically take years to show themselves, and we all eat such a wide
variety of foods that it’s hard to pin down the precise effects of
just one component. Besides, if you were asked to describe exactly
what you’d eaten and drunk in the last week, let alone the last
month, you’d have real problems. Experiments on people are limited
in their scope: would you like to be shut up in a laboratory for six
months while your diet was precisely controlled and its effects on
your bodily functions assessed? Nor would I.

A further problem is that of confounding. People who
take care with their diet and eat ‘healthy’ foods are also more
likely to do healthy things like go jogging, playing tennis and
maintain good family relationships. If you look at the usage of fast
food restaurants, people of low socioeconomic status are going to be
over-represented in the client base, and being poor correlates
negatively with health status generally.

So we don’t have many good reliable data on the
relationship between what we put into our bodies and our healthiness.
Yet at the same time, most people have strongly held intuitive views
about what is a healthy diet and what is not. You can tell how
strongly held these views are when you try to challenge them.

Let’s take an easy target. Fast food. We all know
that eating fast food is unhealthy. A diet of Pizza Hut, MacDonalds,
Burger King and KFC is bad for you. But let me be provocative here.
I’ve got a crazy theory. I reckon you could do a Morgan Spurlock and
just eat fast food for a month, and be perfectly healthy at the end of
it. The fact is, most people who eat fast food all the time get fat
and genuinely are unhealthy. But I reckon this is largely because of
their net consumption—they eat too much—allied with their
lifestyles. In truth, it’s probably very difficult to be healthy on
a diet of just fast food, but this is not because there’s anything
intrinsically toxic about it, but because of the extremely high energy
content of most of the dishes on offer. If Morgan Spurlock hadn’t
supersized his meals, and had excercised more during his thirty days,
I doubt there’d have been much of a documentary.

Our bodies are pretty good at regulating their weight,
but the ‘calculations’ they make are thrown out of step by the
unusually high energy content of Western diets. Thus we pack on the
pounds when we eat what our body tells us is a normal quantity of fast
food. If you had really good self control and under-ate, and you
instituted a program of regular exercise, you could live perfectly
healthily on fast food alone, although I don’t think this a prospect
that appeals to me. But that’s aesthetics. We choose to eat what we
do for perfectly valid reasons other than healthiness.

What do you need from your diet? Some protein, some
carbohydrate, some fat, plus vitamins and trace elements. Perhaps some
fibre, too. The body is pretty versatile, and an otherwise healthy
adult would have to work quite hard in western societies to end up
with a vitamin deficiency, or suffer some other form of malnutrition.
And while enough of these vitamins and trace elements are needed for
health, an excess doesn’t do any good at all.

Why is it that foods such as salad are good for you? It
may well not just be because of anything intrinsically good about
them—yes, they do contain some vitamins and trace elements, but an
excess of these isn’t positive—but rather because of their
displacement effect. If you eat five portions of fruit or vegetables
each day, you are going to be eating less of other higher-calorie
stuff.

A thorough analysis of the science of diet is beyond
the scope of this piece, but let me illustrate the complexity of the
relationship between diet and health with a couple of examples,
beginning with the antioxidant story. Antioxidants such as vitamins C
and E are present in many foods, and in theory should counter damage
to cells from oxidation. This is what they do in experimental
‘test-tube’ trials. The widespread belief was that taking these
antioxidants as dietary supplements should be health-promoting, but in
large and expensive trials they failed. Why? It’s likely to be a
problem of getting the oxidants to the right place at the right time
for them to have their protective effect, known as ‘tissue
availability’.

Look at the wine and health story. Wine is believed to
be good for you, in moderation—at least that’s what large trials
have shown repeatedly. But scientists are still arguing about
mechanisms (how exactly does wine have its protective effect and what
components are involved?), and some are suggesting that the benefit
seen in trials is an artefact of confounding. Then there’s the
really interesting pro- and pre-biotic story. Taking doses of
‘good’ bacteria helps populate your gut with the right sorts of
microbes and is of some health benefit. Perhaps even more exciting is
the finding that certain foods act as pre-biotics, and help make ideal
conditions for these ‘good’ bacteria to grow in.

I hope this short piece has highlighted the complexity
of the relationship between diet and health, and the scientific
uncertainties surrounding it. We tend to have the notion that what
goes into our mouths enters our bodies, and this then leads us to have
a strong intuition that we really are what we eat – that the exact
details of our diet are perhaps more significant in shaping our health
than they really are. This makes us easy to dupe by pseudoscientific
charlatans, and renders us susceptible to the spurious claims of
dietary supplement manufacturers. We also tend to fall for concepts
such as detoxing and colonic irrigation, which have little basis in
reality. Perhaps a more rational understanding of diet, allied with
the notion that much of what we eat stays outside the body in the gut,
might help keep our thinking here.